Electronically commutated motors (ECMs) having two-strand driver circuits with no current limitation, which motors have a predetermined power density, are often used to drive small fans. An increase in the power density of an ECM is usually achieved by improving efficiency. The better the efficiency, the lower the resistance that can, as a rule, be selected for the motor winding of the ECM. The result, however, is that the starting and stalling current of the ECM is many times greater than the average value of the operating current. To prevent overloading of the ECM in this context, corresponding current limiting actions must be taken when the power density of an ECM is raised.
The application of usual current limiting actions to two-strand fan motors requires, however, the use of an electrolytic capacitor in the link circuit, which capacitor ensures, in ON-OFF operation, that the voltage at the link circuit is kept within tolerable limits by the recovery currents in the power stage. Electrolytic capacitors generally have only a limited service life, however, and in the required capacitance range are physically difficult to accommodate. In addition, electrolytic capacitors in the required dimensional range are obtainable exclusively in wire-connected form and—when circuit boards having fully surface-mounted devices (SMDs) are used—cannot be placed onto the boards using the reflow soldering process.